AnswerRocco,
You will need to pull the unit from installation to change out the thermostat. The Round disc at the top of the unit in the center is for the stirrer and you cannot access the thermostat from here.

AnswerEd,
This issue could be caused by a bad thermal fuse, diode, HV capacitor, transformer or the magnetron. You can pull the magnetron and check to make sure that the magnetron magnet is not cracked. Also check the resistance of the magnetron. The resistance should be around .5 ohms. Check the diode. In one direction you should have continuity and the other you would not. If you do then replace the diode. Check thermal fuses for continuity. No continuity means the part is bad.